1. Field of The Invention
The present invention relates to a hollow fiber filter module. More particularly, the present invention is concerned with a hollow fiber filter module comprising a casing having a liquid introducing nozzle at least at one end portion thereof and having a bundle of hollow fibers disposed in the casing, and wherein one end portion of the bundle is enclosed by a hollow, liquid flow control member, and the liquid introducing nozzle has an inlet opening which has a larger diameter than that of the hollow of the nozzle at any portion thereof. The hollow fiber filter module can advantageously be used for separation-purification of liquids, especially in filtration in which a liquid to be filtered is brought into contact with the outer wall surface of a hollow fiber membrane and a filtrate is withdrawn from the inner wall surface of the hollow fiber membrane, i.e., so-called external pressure type filtration. By the use of the filter module of the present invention, the external pressure type filtration of the liquid can advantageously be stably performed for a prolonged period of time with a high filtration efficiency.
2. Discussion of Related Art
When a hollow fiber filter module to be used in the internal pressure type filtration (i.e., filtration in which a liquid to be filtered is brought into contact with the inner wall surface of a hollow fiber membrane and a filtrate is withdrawn from the outer wall surface of the membrane) is used in the external pressure type filtration without any modification of the structure of the internal pressure type filtration module, various problems occur. Particularly, when a filter module comprising a cylindrical casing having a liquid introducing nozzle at one end portion of the casing in its periphery and having a bundle of hollow fibers disposed in the casing along the length thereof is used in the external pressure type filtration without any modification of the structure of the internal pressure type filtration module, problems occur such that the hollow fibers in the bundle are pressed against each other, at a portion of the bundle corresponding to the position of the liquid introducing nozzle, under the great force of a liquid to be filtered flowing into the module through the liquid introducing nozzle, so that the liquid cannot be effectively brought into contract with the hollow fibers which are present within the bundle. Further, the hollow fibers are likely to be broken or otherwise damaged under the force of the liquid to be filtered flowing into the module through the liquid introducing nozzle. These problems are particularly serious when the filter module is relatively large and hence the flow rate of the liquid to be filtered is large. To solve these problems, various proposals have been made. In one proposal, the bundle of hollow fibers is wrapped in a net of a synthetic resin. In another proposal, a baffle plate is disposed between the liquid introducing nozzle and the hollow fiber bundle. In a further proposal, a cylindrical barrier is disposed around the hollow fiber bundle at a portion thereof corresponding to the position of the liquid introducing nozzle.
However, particularly, in the above-mentioned proposals in which a baffle plate or a cylindrical barrier is used, a loss is caused in the pressure to be exerted against the hollow fibers by the liquid introduced through the liquid introducing nozzle, so that the filtration efficiency of the module becomes unsatisfactory. Further, the above-mentioned proposals have a disadvantage in that the module suffers from a lowering of filtration efficiency at the early stage of the operation, depending on the type of the liquid to be filtered. The exact reason for the lowering of the filtration efficiency at the early stage has not yet been elucidated. However, it is presumed that in the modules of the above-mentioned proposals, there is irregularity in the flow rate of the liquid in the circumferential direction so that an accumulation of impurity particles on the outer wall surfaces of the hollow fibers progresses at portions where the flow rate is low. Thus, in the art, there is a strong demand for a hollow fiber filter module which can advantageously be used in the external pressure type filtration without the above-mentioned problems.